The principle of employing infrared radiation to detect the movement of intruders is well-known. The prior art discloses many Passive Infrared Detector apparatuses (hereinafter "PIR") that receive infrared radiation (hereinafter "IR radiation") from a field of view to detect when intruder enters a protected area.
The functioning of PIR detectors is dependent upon a temperature differential between the intruder and the background. An intruder such as a person typically has a higher body temperature, e.g. 37.degree. C., and than the background temperature, e.g. 20.degree. C., thus the difference or contrast between the radiation emitted by the intruder and the ambient radiation (produced by background objects) can be sensed and an alarm triggered when the contrast signal exceeds a specified threshold. As the background and ambient temperatures are nearly typically equivalent, they will be considered for all practical purposes as equivalent and the insignificant difference between them will be neglected. It is accordingly to be understood that in the context of the description and the appended claims the term "ambient temperature" and "background temperature" are interchangeable.
For the range of temperature differences that normally exist between an intruder and background objects, the contrast signal is approximately proportional to the temperature difference between the intruder and background objects normally present in the protected area. In fact, the contrast signal complies with Stephan-Boltzman's law, as will be explained in greater detail below.
The sensitivity (also referred to as detection range) of these detectors is dependent to a large extent on the ambient temperature, i.e. the sensitivity, decreases as the aforementioned contrast level decreases (which, as a rule, occurs when the ambient temperature approaches the intruder body temperature), and hence an infrared detector may not be able to discern an intruder when the temperature thereof nearly matches the ambient temperature.
Environmental conditions where the ambient temperature nearly matches the temperature of the intruder are particularly prone to occur in hot equatorial climates and the like.
Thus, when the contrast level generated by the PIR detector is of relatively low intensity, it had been found advantageous to amplify it by a given amplification gain so as to obtain a sufficient amplitude which is then fed to the alarm circuit which, in turn, activates the "alarm signal" should the amplified contrast signal exceed a pre-determined threshold.
The prior art discloses apparatuses which compensate for the reduced IR detecting sensitivity under the aforementioned environmental conditions (hereinafter "non-discernable intrusion temperature conditions").
There also exist IR-detectors which incorporate an ambient temperature sensor, such as a thermistor or any other temperature sensor, which are adapted to amplify the contrast signal in accordance with the ambient temperature so as to obtain a uniform sensitivity or detection range. Alternatively, the amplification gain may be held invariant and the threshold level which triggers the alarm may be modified in accordance with the ambient temperature so as to maintain the specified uniform sensitivity of detection. U.S. Pat. No. 4,195,234 discloses one such apparatus which delivers an alarm signal when the level of radiation detected changes from the ambient level to a threshold level. A temperature responsive circuit therein adjusts the threshold level so as to decrease the threshold as the ambient temperature increases, or in an alternative embodiment increases the amplification gain as the ambient temperature increases.
The alarm device disclosed in the specified U.S. Pat. No. 4,195,234 failed in attaining the desired uniform sensitivity in particular in the case where the ambient temperature surpasses the intruder temperature. More specifically, the temperature responsive circuit disclosed therein provides an ever increasing amplification gain (or in an alternative embodiment ever decreasing threshold level), as the ambient temperature increases. Bearing in mind that the contrast signal produced at the output of the PIR sensor inherently increases as the ambient temperature rises over the intruder temperature, it appears that the ever-increasing intrinsic sensitivity of the PIR sensor is, needlessly, further enhanced (owing to the ever-increasing amplification gain or ever-decreasing threshold level) in the case where the ambient temperature exceeds the intruder temperature. thereby increasing the probability for undesired spurious alarms (due to radio frequency interference (RFI), electrical transients and others).
Moreover, even in the complementary range, i.e., where the ambient temperature drops below the intruder body temperature, the device disclosed in the specified U.S. Pat. No. 4,195,234 fails in attaining true uniform detection range since the ambient temperature compensation means disclosed therein provides for essentially monotonically increasing amplification gain, whereas the contrast signal decreases as the ambient temperature approaches the intruder body temperature in compliance with the Stephan-Boltzman's Law, i.e. it decreases exponentially to the power of four.